This invention relates to a group electric drive in which the individual operating shafts are coupled to synchronous motors, the motors being being connected to an operating static converter. The output voltage frequency and phase of the converter depends on a control frequency from a frequency generator.
In order to change the angular position of an operating shaft relative to another shaft in a group, a second phase shifting converter is provided, which operates at the same voltage amplitude and is initially connected to the same frequency generator as the operating converter. The synchronous motor whose phase angle position is to be changed is connected to the phase shifting converter and, after being switched to a second frequency generator, the phase angle is changed by varying the frequency of the second frequency generator in the direction of the desired change.
After the desired angle position is reached, the phase shifting converter can again be connected to the frequency generator of the operating converter if the phases of the output voltages of the converters as determined by an appropriate detector agree. Subsequently, the synchronous motor is connected back to the operating converter.
Thus, in this arrangement, the additional converter serves for correcting the phase angle positions of individual motors with respect to the phase angle position of a lead motor.
Often it is required that relatively large motors be connected to a converter network operating, for instance, at a frequency of 150 Hz. To make such a switching action permissible the power rating of the operating converter described above would have to be increased substantially. Furthermore, excessive thermal stressing of the motors, due to slip losses, may occur.
It is an object of the present invention to provide a device of a type described above in which the addition or removal of motors to or from a group running at the operating frequency can be accomplished at no extra cost.